1. Field of the Invention
The present invention relates in general to a resistor element having an electrically resistive thin film, and more particularly to such a resistor element which is suitably used for a temperature sensor or a thermal flow meter, for example.
2. Discussion of the Prior Art
An example of a known resistor element of the above type is shown in FIG. 1. The resistor element includes a ceramic tube 2 formed of alumina, for example, and a thin metallic film 4 which is made of platinum or other metal, and which is formed on the outer circumferential surface of the tube 2. The metallic film 4 is suitably patterned so as to have a desired resistance value. The patterned metallic film 4 is electrically connected at the longitudinally opposite ends of the ceramic tube 2, to a pair of lead wires 6, 6 made of platinum, for example, by respective electrically conductive connectors 8. The connectors 8 are obtained by mixing an electrically conductive material, such as platinum, with glass, for example.
In the known resistor element of the above type, the electrically conductive connectors 8 for connecting the lead wires 6 to the metallic film 4 are also used for securing the lead wires 6 to the ceramic tube 2. However, the use of the connectors 8 undesirably pushes up the cost of fabricating the resistor element, since the connectors 8 contain a relatively large amount of a rather expensive electrically conductive material, such as platinum.
In view of the above drawback, a method as illustrated in FIG. 2 is proposed for fabricating the resistor element. According to the method, an end portion of each lead wire 6 is inserted a suitable distance into the corresponding end portion of an central bore of the ceramic tube 2, and is bonded to the inner circumferential surface of the tube 2 by a suitable adhesive 10, such as glass. Then, an electrically conductive paste for the connector 8 is applied to the corresponding end face of the ceramic tube 2, so that the lead wire 6 is electrically connected by the connector 8 to the metallic film 4 formed on the outer circumferential surface of the ceramic tube 2.
In the resistor element of the type shown in FIG. 2, however, the metallic film 4 has to be electrically connected to the lead wires 6 by the electrically conductive connectors 8, after the lead wires 6 are secured by bonding to the ceramic tube 2 by the adhesive 10. Thus, the above-described method requires an increased number of fabricating steps to obtain the intended resistor element, and eventually pushes up the cost of the element. Further, if external forces act on the lead wires 6 while the electrically conductive paste is applied, dried and then heat-treated to form the connector 8 in the process of fabricating the resistor element, or while the lead wires 6 are handled during practical use of the element, the connector 8 is subjected to stress, and suffers from cracks caused by the stress. Thus, the known resistor element constructed as described above has relatively low operating reliability and undesirably reduced durability.